Polyethylene (PE) is synthesized by polymerizing ethylene (CH2═CH2) monomers. Because it is cheap, safe, stable to most environments and easy to be processed polyethylene polymers are useful in many applications. According to the properties polyethylene can be classified into several types, such as but not limited to LDPE (Low Density Polyethylene), LLDPE (Linear Low Density Polyethylene), and HDPE (High Density Polyethylene). Each type of polyethylene has different properties and characteristics.
Ethylene polymerizations are frequently carried out in a loop reactor using ethylene monomer, liquid diluent and catalyst, optionally one or more co-monomers, and hydrogen. The polymerization in a loop reactor is usually performed under slurry conditions, with the produced polymer usually in a form of solid particles which are suspended in the diluent. The slurry in the reactor is circulated continuously with a pump to maintain efficient suspension of the polymer solid particles in the liquid diluent. Polymer slurry is discharged from the loop reactor by means of settling legs, which operate on a batch principle to recover the slurry. Settling in the legs is used to increase the solids concentration of the slurry finally recovered as product slurry. The product slurry is further discharged through heated flash lines to a flash tank, where most of the diluent and unreacted monomers are flashed off and recycled.
Alternatively, the product slurry may be fed to a second loop reactor serially connected to the first loop reactor wherein a second polymer fraction may be produced. Typically, when two reactors in series are employed in this manner, the resultant polymer product is a bimodal polymer product, which comprises a first polymer fraction produced in the first reactor and a second polymer fraction produced in the second reactor, and has a bimodal molecular weight distribution.
After the polymer product is collected from the reactor and the hydrocarbon residues are removed therefrom, the polymer product is dried, additives can be added and finally the polymer may be extruded and pelletized.
During the extrusion process ingredients including polymer product, optional additives, etc, are mixed intimately in order to obtain a compound as homogeneous as possible. Usually, this mixing is done in an extruder wherein the ingredients are mixed together and the polymer product and optionally some of the additives are melted so that intimate mixing can occur. The melt is then extruded into a rod, cooled and granulated, e.g. to form pellets. In this form the resulting compound can then be used for the manufacturing of different objects.
Polymerization of ethylene involves the polymerization of ethylene monomer in the reactor in the presence of a polymerization catalyst and optionally, if required depending on the used catalyst, an activating agent. Suitable catalysts for the preparation of polyethylene comprise chromium catalysts, Ziegler-Natta catalysts and metallocene catalysts. Typically, the catalyst is used in particulate form. The polyethylene is produced as a resin/powder with a hard catalyst particle at the core of each grain of the powder.
Several systems have been disclosed which involve the preparation and the supply of catalyst slurry to a polymerization reaction. In general, for preparing catalyst slurry, a mixture of dry solid particulate catalyst and diluent are apportioned in a catalyst mixing vessel and thoroughly mixed. Then such catalyst slurry is typically transferred to a polymerization reactor for contact with the monomer reactants.
It is known in the art that for the production of ethylene polymers having suitable properties it is important during polymerization to control reaction conditions, including reaction temperatures, reactant concentration, etc. Polymerization reactions are also sensitive to the quantity, quality and the type of catalyst utilized. Sub-optimal conditions at the start of or during the polymerization reaction may lead to a sub-optimal polymerization conditions resulting for instance in low production yields and/or the production of polymers having undesired properties and/or falling off specifications. In view thereof, ethylene polymerization reactions require accurate and adaptive monitoring and control of the reaction conditions.
In particular, the concentration of a particulate catalyst in a diluent has a direct and immediate effect on polymer characteristics such as polymerization product granulometry and polymerization product particle density, as well as on polymerization characteristics such as polymerization processivity. Therefore, a change in catalyst concentration has a profound effect on various polymerization parameters and hence the final polymer product. Indeed, (local) catalyst concentration differences in a polymerization reaction result in unwarranted polymer heterogeneity in respect of for instance product density, granulometry and molecular weight (distribution).
As catalyst slurries comprise a solid particulate catalyst suspended in a liquid diluent, such slurries are prone to sedimentation. Adequate mixing of the catalyst slurry is needed to assure a homogeneous distribution of the solid catalyst particles in the diluent before the catalyst slurry is fed to the polymerization reactor.
Moreover, the physicochemical characteristics of the catalyst well as of the diluent, including for instance the type of catalyst and diluent, the specific gravity of the catalyst, catalyst granulometry, catalyst settling velocity, catalyst concentration, the viscosity of the diluent and the catalyst slurry as well as the desired properties of the polymerization product require a highly flexible and adaptable catalyst slurry preparation and mixing system for adequately preparing a diluted catalyst slurry.
In view of the above, there remains a need in the art to provide an improved catalyst preparation system for preparing a diluted catalyst slurry with suitable properties for use in a polymerization process for making polyolefin resin, and in particular polyethylene.